**3.2 Preventive winemaking practices to avoid white wine protein instability**

Regarding the mechanisms of wine protein turbidity development, there are numerous potential approaches for avoiding wine turbidity that would either decrease or remove the requirement for bentonite application. These comprise reducing the concentration of wine phenolic compounds; reducing the wine ionic strength; disrupting hydrophobic protein-protein interactions; stabilising wine proteins against thermal unfolding; degrading wine proteins enzymatically after heat treatment; application of alternative adsorbents or ultrafiltration to eliminate proteins [9].

Enzymes application to degrading haze-forming proteins in wine is a specially an attractive substitute to bentonite since it diminishes aroma removal and wine losses. Preferably, active enzymes would be applied to grape must without the requirement for future removals, as in the case of glucanases and pectinases [78]. The products of grape proteins degradation may also be used by yeast as nitrogen sources, theoretically decreasing the common necessity for nitrogen application and enhancing wine aroma quality [79, 80]. For wine protein degradation, there are two important kinds of enzymatic activities: the decrease of disulphide bonds by protein disulphide reductases and the hydrolysis of peptide bonds by proteases [81]. The difficulty in using proteases for specifically degrading haze-forming proteins in wine is related to the stability of the proteins in wine-like conditions. Protein disulphide reductases could, hypothetically, destabilise and precipitate hazeforming proteins throughout vinification via reduction of disulphide bonds [22]. Nevertheless, under wine conditions, there have been no published cases of active protein disulphide reductases.

In a Champenois Chardonnay wine, it has been shown that a 24/25 kDa protein was an N-glycosylated protein and underwent no modification throughout fermentation [82], whereas degradation or variation of the sugar moieties of the glycoproteins (12–30 kDa) was found to happen during winemaking for a hybrid grape variety (Muscat Bailey A) [83]. The hydrolysis of the sugar chains of grape derived glycoproteins by glycosidase treatment was found to rise turbidity with seed phenols in a model wine [84]. Instead, yeast-derived mannoproteins (420 and 31.8 kDa) could contribute to a stabilisation effect on wine proteins, decreasing haze development [85, 86]. Yeast derived mannoproteins (10–30 kDa) possessing both compositions of the hydrophobic and hydrophilic protein domains and mannose moiety also improved the foaming properties in sparkling wines [87, 88].

Another strategy to decrease the level of proteins in white wines is pre-fermentative skin maceration, for example, in the Albariño grape variety, pre-fermentative skin maceration augmented the concentration of polysaccharides and phenolic compounds extracted, however, reduce the quantity of protein extracted, mainly of the pathogenesis-related proteins, specifically the *V. vinifera* chitinases and thaumatin-like proteins. While the PRPs and total protein of the Albariño wine produced by pre-fermentative skin maceration were lesser, the wine presented higher protein instability in the heat test, perhaps the presence of higher level of polyphenols compounds [17].
